The purpose of this proposal is to develop a novel method of fragmentation of biomolecules in an electrospray ionization (ESI) quadrupole ion trap (QIT) mass spectrometer (MS) using an electron capture dissociation (ECD) technique. In experiments with polypeptide ions conducted in an ESI ion cyclotron resonance Fourier transform mass spectrometer (ICR-FTMS), which is another type of an ion trap, ECD has been shown to cleave many more bonds than a conventional collisionally activated dissociation (CAD) technique. Unlike the latter, ECD preservs labile modifications of the amino acid side chains, such as phosphorylation and glycosylation, that allows for easy determination of their sites, can distinguish between Leucine and Isoleucine amino acid residues in peptide sequences and produces no rearrangement ions, thus, simplifying peptide sequencing. The major difference of QITMS from ICR-FTMS is the presence of a strong oscillating electric field that complicates the introduction of low energy (less than 0.5 eV) electrons into the trap. Our approach is directed toward designing an RF trapping potential having a negligible impact on the energy of the injected electrons. Given the exceptional ECD fragmentation properties, the proposed approach may become a method of choice for dissociation of biomolecules inside QITMS in the future.